A piezoelectric actuator, also denoted as piezoactuator, is constructed with the aid of piezoelectric material and serves as mechanical drive for producing a movement by varying a dimension, specifically the length, of the piezoactuator by means of an electric voltage. A piezoactuator is typically constructed as a piezostack in which layers of a piezoelectric material of alternating polarization directions alternate with metal electrodes. The electrodes are connected alternately to one of two outer electrodes in the sequence of their arrangement.
An electric AC voltage applied to the outer electrodes generates a longitudinal vibration in the longitudinal direction of the piezostack, that is to say in the direction in which the layers of the piezostack follow one another. A longitudinal vibration of the piezoactuator is used in a multiplicity of applications, for example for the purpose of fuel injection. To this end, the piezostack is mounted in its longitudinal direction between holders or accommodating components. Usually, one or more resonance frequencies occur at which the excited longitudinal vibration is particularly strong.
Transverse vibrations which likewise occur are, by contrast, undesired because they cause distortions of the piezoactuator that impair the function of the piezoactuator and, in some instances, deleterious cracks in the edge surfaces of the piezostack. The resonant frequencies of the transverse vibrations are generally much lower than the resonant frequencies of the longitudinal vibrations. For example, the resonant frequency of the longitudinal vibration is typically approximately 12 kHz in the case of a 30 mm piezoactuator while, depending on the mounting of the piezoactuator, transverse vibrations can already occur at 2 kHz to 3 kHz and be excited by signals of relatively low frequency.
Ideally, on the basis of the symmetry, usually present, in the structure of a piezoactuator, no transverse vibrations should occur when the piezoactuator is mounted symmetrically in the holders. However, in practice it is not always possible to exclude a low degree of asymmetry in the mounting of the piezoactuator, nor a mechanical distortion resulting therefrom. In addition, an asymmetry can be present inside the piezoactuator because of manufacturing tolerances, and this likewise favors the occurrence of transverse vibrations.
During mounting or assembling, a most symmetrical mounting is ensured by measurements of the geometric alignment of the piezoactuator in the holders. However, it is not thereby possible to offset an asymmetry present in the piezoactuator itself. In addition, this mounting method does not permit any assessment of the actual inclination of the piezoactuator in relation to flexural vibrations. It is likewise impossible in this way to monitor the occurrence of flexural vibrations during operation of the piezoactuator, in particular a lengthy time after first commissioning, which can be desired in the case, for example, of a piezoactuator which is installed in an engine provided for a motor vehicle.